This invention relates to a tire wheel capable of preventing bead portions from falling from bead seats onto a well when tire inner pressure lowers. It is also capable of easily mounting and dismounting a tire onto and from the tire wheel with the aid of a hump provided on the tire wheel.
Tire wheels having humps described above have been known, one of which is shown in FIG. 1 by way of example. The hump 52 of this tire wheel 51 shown in FIG. 1 includes on both sides of a crest 53, a seat side inclined surface 55 on the side of a bead seat 54 and a well side inclined surface 57 on the side of a well 56. The seat side inclined surface 55 is composed of a circular arc surface radially outwardly convexed and having a radius Rd with a center D and a circular arc surface radially inwardly concaved and having a radius Re with a center E. On the other hand, the well side inclined surface 57 is also composed of a circular arc surface having the radius Rd and a circular arc surface radially inwardly concaved and having a radius Rf with a center F.
In this case, as the seat side inclined surface 55 of the hump 52 is composed of the two smoothly transitional circular arc surfaces, a bead portion B of a pneumatic tire T easily rides over the hump 52 to fall onto the well 56 when the inner pressure of the pneumatic tire T lowers owing to puncture or the like, while the pneumatic tire T is subjected to a large lateral or transverse force in turning of a vehicle equipped with the tire. This is a problem of such a wheel to be solved by this invention.
Moreover, a tire wheel 61 has been proposed as shown in FIG. 2. This tire wheel 61 includes a circular arc surface 65 radially inwardly concaved from a top 63 of a hump 62 onto the side of a bead seat 64 and having a radius Rg with a center G, a relatively long flat surface 67 (usually of the order of 4 to 5 mm) extending toward a well 66, and a circular arc surface 68 continuous to the well 66 and radially outwardly convexed having a radius Rh with a center H. However, this tire wheel encounters the same problem as described above.
In order to solve such a problem, for example, tire wheels as shown in FIGS. 3 and 5 have been proposed. With the tire wheel 71 shown in FIG. 3, the sectional shape of a hump 72 is changed depending upon its circumferential positions. The sectional shapes of the hump 72 are respectively shown in solid lines at the circumferentially 0.degree. position, in a dash line at the circumferentially 90.degree. and 270.degree. positions, and in a two-dots-and-dash line at the circumferentially 180.degree. position. In other words, this shape of the tire wheel is formed by progressively changing inclined angles of a surface P around an axially constant position K correspondingly to circumferential positions of the surface P. A seat side inclined surface 73 is inclined at a large angle to a tire rotating axis at the circumferentially 0.degree. position, but inclined at a small angle to the rotating axis at the circumferentially 180.degree. position.
On the other hand, a tire wheel 81 shown in FIG. 5 includes a seat side inclined surface 83 of a hump 82 arranged at substantially perpendicular to a tire rotating axis or extending substantially radially outwardly.
With the tire wheel 71 shown in FIG. 3, however, the inclined angles of the seat side inclined surface 73 change depending upon circumferential positions as shown in FIG. 4. Therefore, when inner pressure in a pneumatic tire T mounted on the tire wheel 71 lowers, while the pneumatic tire is subjected to a lateral force in a direction shown by an arrow in FIG. 4, the bead portion of the tire T behaves in the following manner. In the proximity of the circumferentially 0.degree. position where the inclined angle of the seat side inclined surface 73 is large, the bead portion engages the seat side inclined surface 73 to make movement of the bead portion difficult toward the well 74. However, in the proximity of the circumferentially 180.degree. position where the inclined angle of the seat side inclined surface 73 is small, the bead portion can move easily toward the well 74.
Once a circumferential portion of the bead portion B (bead portion at the 180.degree. position) has deformed and ridden on the seat side inclined surface 73 in this manner, the deformed and ridden portion progressively increases as road contacting portion of the rolling tire moves along its circumference and the tire is subjected to the lateral force until the bead portion falls onto the well 74. Although this tire wheel 71 can improve the prevention of the bead portion B from falling onto the well 74 to a some extent in comparison with the tire wheel 51 shown in FIG. 1, it is not sufficient to use the tire wheel 71 for high performance tires which have been recently developed. Moreover, there is a problem of the tire wheel 71 being peculiar in configuration so that forming thereof is difficult and expensive.
With the another tire wheel 81 shown in FIG. 5, on the other hand, the seat side inclined surface 83 of the hump 82 extends substantially perpendicular to the tire rotating axis as shown in FIG. 6. Therefore, when the inner pressure in the pneumatic tire T lowers and the tire is subjected to a lateral force in a direction shown by an arrow in FIG. 6, a bead toe C of a bead portion corresponding to a road contacting portion engages the seat side inclined surface 83 so that the bead toe C is subjected to a great shearing force. As a result, the bead toe C is partially damaged (bead toe chipping) so that there is a risk of carcass cords and the like of the bead portion being exposed.
Moreover, this tire wheel 81 encounters a difficulty in that bead toe chipping would occur when the tire T is dismounted from the tire wheel 81 because of the fairly large force which is required in dismounting of the tire from the wheel 81.
In order to solve these problems, the inventors of this application proposed a tire wheel 91 shown in FIG. 7 as disclosed by a copending U.S. patent application Ser. No. 07-456,911 which is incorporated herein by reference. In this tire wheel 91, the height H of a crest 93 of a hump 92 (one half of difference between the diameter of the crest 93 and the actual diameter of a rim) is set 0.5 mm to 2.5 mm. On both sides of the crest 93 the hump 92 is formed with an inclined surface 96 on the side of a bead seat 94 and an inclined surface 97 on the side of a well 95. The inclined surface 96 on the side of the bead seat 94 is inclined at an angle of about 5.degree. to 30.degree. to a straight line in parallel with a tire rotating axis, while a concaved corner 98 having an obtuse angle is formed at a boundary between the inclined surface 96 and the bead seat 94.
With this tire wheel 91, when inner pressure in a pneumatic tire T lowers and the tire T is subjected to a lateral force, a tire toe C rides over the corner 98 having the obtuse angle and at the same time the entire circumferential surface of a bead portion B of the tire T completely rides on the inclined surface 96. Therefore, as the bead portion B approaches the crest 93 of the hump 92, the bead portion B exhibits a so-called "hoop effect" so that the bead portion B is fixed to the inclined surface 96. Consequently, the bead portion B is prevented from falling onto the well 95 in this manner. This tire wheel 91 already proposed by the inventors is very superior to those of the prior art in the prevention of bead portion from falling onto the well.
However, this tire wheel 91 has room for more improvement. In more detail, in case that the inclined angle of the inclined surface 96 on the side of the bead seat 94 (with respect to the tire rotating axis) is less than about 10.degree., a distance A between the corner 98 having the obtuse angle and the crest 93 of the hump 92 at the top of the inclined surface 96 becomes too long. Such a long distance A makes difficult the tire mounting and dismounting operation.
In order solve this problem, the inventors of this application further investigated the distance A between the crest 93 of the hump 92 and the corner 98 in many experiments. As a result, they have found that the distance A of 15 mm to 16 mm is an upper limit to avoid the difficulty in tire mounting and dismounting operation, and a distance A longer than this value exhibits too large a hoop effect at a bead portion so that more consideration is required in actual application.
On the other hand, the preferable lower limit of the height H of the crest 93 of the hump 92 is of the order of 1 mm in consideration of prevention of the bead portion from falling onto the well. In connection with the height H and the tire mounting and dismounting operation, the inclined angle of the inclined surface 96 becomes somewhat larger than 10.degree. in order to maintain the distance A about 15 mm and the height H approximately 1 mm. Even if it is attempted to set an inclined angle of 10.degree. and a height H of 1.5 mm as a desirable mean value, the distance A becomes about 19 mm which makes difficult the tire mounting and dismounting operation.
As can be seen from the above explanation, although the tire wheel 91 already proposed by the inventors of this application is very effective to prevent the bead portion from falling onto the well in running with lowered inner pressure, it includes a difficulty in that freedom in the selection of a combination of the distance A and the height H is limited to a narrow range in consideration of the tire mounting and dismounting operation. Moreover, a width P of the bead seat 94 is usually determined by various standards, for example, more than 20 mm for passenger cars. In the case that bead bases of a tire are wide, the width P often exceeds 20 mm. In such a case, the relation between the height H and the distance A is further limited so that the freedom in design of the tire wheel is limited to a narrower range.